Alkaline batteries, in which an alkaline electrolyte is used, and lead-acid batteries are widely used as rechargeable batteries to serve as various power sources. Generally, alkaline batteries have a long service life and are highly reliable, compact and lightweight. For those and other reasons, small-size cells and batteries are now rapidly extending their range of application in various portable devices and apparatuses, whereas large-capacity, large-size batteries have a long history as industrial power sources.
A number of battery systems are available for alkaline batteries. While some systems use an air electrode or a silver oxide electrode as the positive electrode, most of the alkaline battery systems recently available use, as the positive electrode, a nickel electrode comprising nickel oxides as active materials. The original nickel-based positive electrode was a long-life pocket type plate. Later advent of the technology of sintered plates, which show high rate charge and discharge characteristics, and of foamed metal type plates, which have a higher capacity density, has contributed to a greatly expanded range of use of nickel positive electrodes.
On the other hand, active materials for negative electrodes include not only cadmium but also zinc, iron annd hydrogen, among others. So far, cadmium electrodes have been used in the main since they have a long service life and facilitate sealing of batteries. In particular, the demand for sealed nickel-cadmium rechargeable batteries is rapidly growing with the recent improvements in fast charge characteristics and in capacities. However, the conventional battery systems have their limit in meeting the demand for batteries having a higher energy density for size and weight reduction of devices and apparatuses. Under these circumstances, sealed rechargeable batteries of a nickel-metal hydride system (commonly, nickel-hydrogen system) in which a hydrogen-absorbing alloy higher in capacity density than cadmium and capable oof repeated absorption and desorption of hydrogen upon charging and discharging, respectively, is used as the negative electrode, are reaching the stage of practical use.
Sealed rechargeable batteries employing a nicekl-hydrogen system have a nominal cell voltage of 1.2 V, which is the same as that of nickel-cadmium system batteries. With the same cell size and construction, their discharge capacity can be at least 140% of that of nickel-cadmium system batteries. Thus, the nickel-hydrogen system is advantageous in that it is higher in energy density, and further in that it is compatible with the nickel-cadmium system or, in other words, can be handled in the same manner.
For the manufacture of sealed rechargeable batteries which employ a nickel-hydrogen system, it is basically necessary to establish a positive and negative electrode capacity relationship in the same manner as in the nickel-cadmium system. Thus, preliminary adjustment has to be made to meet the following prerequisites: (1) the negative electrode capacity should be larger than the positive electrode capacity and (2) after charging in the sealed cell state until full charging of the positive electrode, there should exist, in the negative electrode, portions in the uncharged state, which are further chargeable, and portions in the already charged state. When the sealed rechargeable batteries thus adjusted are charged to the extent of overcharge, oxygen is generated from the positive electrode. This oxygen reacts with, and is absorbed into, the charged-state portions of the negative electrode to give portions in the uncharged state. Accordingly, further continued charging will not result in full charging of the negative electrode. Thus, in principle, hydrogen will not be generated from the negative electrode forever. The oxygen cycle reaction mentioned above establishes and maintains an equilibrium pressure within the cell. To cause portions in the prelimiinary charged state to exist in the negative electrode is to make the dischargeable capacity of the negative electrode greater than that of the positive electrode. In this arrangement, the discharge capacity of the cell is limited by the positive electrode even in deep high-rate discharge and the unrepairable damage due to overdischarge and passivation of the negative electrode is precluded. Moreover, the existence of such a precharged portion of the negative electrode means that the negative discharge still has a substantial reduced portion even when the positive electrode has been fully charged to generate oxygen and, therefore, the oxygen absorbing capacity of the negative electrode is increased. Therefore, byy increasing the precharged portions of the negative electrode, an improvement can be obtained in the fast charge characeristics which is one of the important qualities of a sealed battery. As aforesaid, provision of precharged portions in the excess capacity of the negative electrode over the positive electrode is not only conducive to improved discharge rate characteristics and suppression of capacity decrease of the negative electrode, but also contributes to improved fast charge characteristics. The uncharged portion of the capacity of the negative electrode as the balance after such pre-charging in the excess capacity over the positive electrode insures no evolution of hydrogen from the negative electrode even in an overcharged state, contributing to inhibition of pressure buildup within the cell.
Establishment of such a capacity appropriation between the positive and negative electrodes is the basic requisite for implementing an improved sealed battery. Other important parameters are electrode plate group configuration or geometry, selection of the proper separator, proper electrolyte composition and volume, and so on.
In achieving said proper appropriation of capacity between the positive and negative electrodes, it is an important procedure to pre-charge the negative electrode to a given extent to partially reduce the same. In the conventional nickel-cadmium system, whereas the positive electrode is eitehr an uncharged/unformed electrode or an electrode subjected to thorough formation, viz. full charging and full discharging, the negative electrode is prepared by a formation process in which an unformed electrode plate or an electrode plate previously charged and discharged is partially charged to a required extent. The positive and negative electrodes thus formed are rinsed, dried and assembled with a separator into an electrode plate group to provide a sealed battery.
Referring to the nickel-hydrogen system, U.S. Pat. No. 4,716,088 describes an example in whichh, although the object is to activate the hydrogen absorbing alloy of the negative electrode, the negative electrode is caused to absorb hydrogen by charging to electrochemically reduce it to the hydride (hydriding) just as in the case of the cadmium electrode. Since an electrode plate group is fabricated using such a partially charged and formed hydrogen absorbing alloy as the negative electrode, withdrawal from the alkaline electrolyte, rinsing and drying causes oxidation of the negative electrode on contact with the atmospheric air and at times a violent oxidation reaction occurs to cause a problem of ignition. If short of ignition, hydrogen is readily desorbed from the negative electrode under atmospheric pressure and even if the storage atmosphere is replaced with hydrogen taking the equlibrium pressure into consideration, it is extremely difficult to control the precharged portion of the negative electrode exactly as designed.
Therefore, an alternative approach has been taken to insure an excess capacity of the negative hydrogen absorbing alloy electrode over the positive electrode when the positive electrode has been fully charged. Thus, in a non-sintered plate such as aone of the foamed metal type, cobalt is added to the active material, nickel hydroxide, for improved utilization rate, not using the conventional cobalt hydroxide alone but using finely divided powders of cobalt metal powder, like carbonyl cobalt. Using such a positive electrode, a negative hydrogen absorbing alloy electrode having a larger capacity than the positive electrode and a separator, an electrode plate group is assembled and after the electrode plate group is inserted in a cell housing, a predetermined quantity of the electrolyte is added. After sealing, the assembled battery is charged. By this charging, the metal cobalt in said positive electrode is oxidized together with the active nickel hydroxide and contributes to an improved utilization rate of the positive active material alongside the cobalt hydroxide separately added, but as is well known the cobalt oxide as such does not substantially function as a positive active material.
Thus, by the amount in which the metal cobalt added to the positive electrode is oxidized by charging, the proportion of the charged state of the negative electrode is increased. First, the charged portion of the negative electrode can be increased by increasing the level of addition of metal cobalt to the positive electrode, but there is a limit to this practice in view of insuring a high capacity of the cell. Moreover, the use of metal cobalt for cobalt supplementation is costly. In addition, since cobalt in metallic form cannot be filled into the nickel plaque in the active material impregnation process, the practice cannot be applied to sintered positive electrodes.
In accordance with the prsent invention, the property of the hydrogen absorbing alloy is utilized to control the relative capacity of the positive and negative electrodes over a wide range regardless of the kind of positive electrode, without being limited to nickel based positive electrodes.